Conjugated Dienes
Chapter 14
Organic Chemistry, 8th Edition
John E. McMurry
1
Dienes
• Propadiene (allene) is a cumulated diene
• 1,3-Butadiene is a conjugated diene.
• 1,4-Pentadiene is an isolated diene.
Allene –
A cumulated diene
H 2C C CH 2
2
Allenes
Bonding
Stereochemistry
Energetics
ΔHf°
(Kcal/mol)
sp
H
H
H H
• Chiral
• 1,3 disubstituted
allenes have no plane
of symmetry
• Axial chirality
H 2C C CH 2
45.5
HC C CH3
44.2
3
Conjugated Dienes
• The C—C single bond joining the two double bonds is unusually short.
• Conjugated dienes are more stable than similar isolated dienes.
ΔHhydrog
- 30
- 60
- 56
(kcal/mol)
• Some reactions of conjugated dienes are different than reactions of
isolated double bonds.
• Conjugated dienes absorb light at longer wavelengths than alkenes.
4
Delocalisation, Resonance, and Dienes
• Delocalisation occurs whenever p orbitals can overlap on three or more
adjacent atoms.
5
Delocalisation, Resonance, and Dienes
3 nodes
ANTIBONDING
Energy
2 nodes
1 node
BONDING
0 nodes
6
Delocalisation, Resonance, and Dienes
• The allyl carbocation is another example of a conjugated system.
• Conjugation stabilizes the allyl carbocation.
7
Structure
• Three stereoisomers are possible for 1,3-dienes with alkyl groups bonded
to each end carbon of the diene.
• Two possible conformations result from rotation around the C—C bond
that joins the two double bonds.
8
Structure
• Stereoisomers are discrete molecules, whereas conformations
interconvert.
9
Conjugated Dienes and Ultraviolet Light
• When electrons in a lower energy state (the ground state) absorb light
having the appropriate energy, an electron is promoted to a higher
electronic state (excited state).
10
Conjugated Dienes and Ultraviolet Light
• The absorption of ultraviolet (UV) light by a molecule can promote an
electron from a lower electronic state to a higher one.
• Ultraviolet light has a shorter wavelength (and thus higher frequency) than
visible light.
• The most useful region of UV light for this purpose is 200-400 nm.
11
Conjugated Dienes and Ultraviolet Light
π∗
π∗
E=hc/λ
π
π
π4
π∗
π3
π2 E’=hc/λ’
π
π1
λmax < 200 nm
12
Conjugated Dienes and Ultraviolet Light
• As the number of conjugated π bonds increases, the energy difference
between the ground and excited state decreases, shifting the absorption
to longer wavelengths.
• With molecules having eight or more conjugated π bonds, the absorption
shifts from the UV to the visible region, and the compound takes on the
color of the light it does not absorb.
13
Conjugated Dienes and Ultraviolet Light
• Lycopene absorbs visible light at λmax = 470 nm, in the bluegreen region of the visible spectrum. Because it does not
absorb light in the red region, lycopene appears bright red.
β-carotene
14
Electrophilic Addition: 1,2- Versus 1,4-Addition
15
Electrophilic Addition: 1,2- Versus 1,4-Addition
16
Kinetic Versus Thermodynamic Products
• The amount of 1,2- and 1,4-addition products formed in electrophilic
addition reactions of conjugated dienes depends greatly on the reaction
conditions.
0 °C
71%
29%
40 °C
15%
85%
17
Kinetic Versus Thermodynamic Products
• When a mixture containing predominantly the 1,2-product is heated, the
1,4-addition product becomes the major product at equilibrium.
18
Kinetic Versus Thermodynamic Products
19
Kinetic Versus Thermodynamic Products
• The 1,2-product is the kinetic product because:
a) the charge on C2 is higher
b) a proximity effect.
20
The Diels-Alder Reaction
• Because each new σ bond is ~20 kcal/mol stronger than a π bond that is broken,
a typical Diels-Alder reaction releases ~40 kcal/mol of energy.
21
Rules for The Diels-Alder Reaction
1. Electron-withdrawing substituents in the dienophile increase the reaction
rate.
• The diene acts as a nucleophile and the dienophile acts as an electrophile.
• Electron-withdrawing groups make the dienophile more electrophilic
Z : electronwithdrawing group
22
Rules for The Diels-Alder Reaction
2. The diene can react only when it adopts the s-cis conformation.
This rotation is prevented in cyclic alkenes.
23
Rules for The Diels-Alder Reaction
3. The stereochemistry of the dienophile is retained.
24
Rules for The Diels-Alder Reaction
4. When endo and exo products are possible, the endo product is
preferred.
25
Rules for The Diels-Alder Reaction
26
The Diels-Alder Reaction in Organic Synthesis
• Steroids are tetracyclic lipids.
27
The Diels-Alder Reaction in Organic Synthesis
O
O
O
28
The Diels-Alder Reaction in Organic Synthesis
29